1. Field of the Invention
The present invention relates to a flat cathode-ray tube and, more particularly, to a curved surface structure of a shadow mask located at the inner side of a panel to select colors of electron beams to allow the electron beams to correctly impact on corresponding fluorescent materials.
2. Description of the Related Art
As shown in FIG. 1, a conventional cathode-ray tube includes a panel 1 having red, green and blue fluorescent materials coated on the inner side thereof, a funnel 2 fused to the panel 1 in the rear of the panel to maintain a vacuum state inside the cathode-ray tube, a tube-shaped neck 10 extended from the back of the funnel 2, an electron gun 8 being inserted in the neck 10 to emit electron beams 11, and a deflection yoke 9 for deflecting the electron beams. The cathode-ray tube further has a reinforcing band 12 for preventing explosion of the vacuum state therein and a lug 13 for fixing the cathode-ray tube, which are located on the outer surface thereof.
A shadow mask 3 is fixed to a frame 4 near the fluorescent materials coated on the inner surface of the panel 1. This shadow mask 3 selects colors of the electron beams emitted from the electron gun 8. The frame 4 is fit in a stud pin 6 set at the inner side wall of the panel by a support spring 5 fixed to the frame. An inner shield 7 is combined with the frame at one side of the frame 4 so that the electron beams toward the fluorescent materials are not affected by external magnetism.
The shadow mask 3 having a predetermined curvature is located at the inner side of the panel, having a predetermined distance from the inner surface of the panel. The shadow mask makes the electron beams 11 emitted from the electron gun 8 reach the red, green and blue fluorescent materials correctly. The curvature of the shadow mask is designed to allow the electron beams to have a uniform distribution corresponding to their arrangement (interval) according to the color selection characteristic. The curvature of the shadow mask is represented by grouping rate (G/R) of electron beams that determines color purity of image.
Referring to FIG. 2, the grouping rate is expressed as follows.
G/R=(3*S*Q)/(Ph*L) xe2x80x83xe2x80x83(1) 
where S is the distance between the center of the electron beams and deflection center that is a base height at which the deflection yoke deflects the electron beams, Q is the distance between the shadow mask and the inner surface of the panel, Ph is a horizontal pitch of the shadow mask, meaning the distance between holes of the shadow mask, and L is the distance between the inner surface of the panel and the deflection center.
Characteristics of the cathode-ray tube, affected by the grouping rate of the electron beams, include purity characteristic such as purity margin and direction change margin. The purity margin means a location allowance of the deflection yoke that does not allow the electron beams to make a fluorescent material at a wrong position radiate due to the location of the deflection yoke 9 so that the electron beams 11 emitted from the electron gun 8 pass through the shadow mask 3 to correctly reach the red, green and blue fluorescent materials. This purity margin facilitates a process of adjusting the screen of the cathode-ray tube.
Meantime, the path of the electron beams 11 is changed under the influence of an external magnetic field (earth magnetic field) when the location of the cathode-ray tube is turned. The direction change margin means an allowable direction change angle that prevents radiation of a fluorescent material that is not a target.
The grouping rate of the electron beams and the horizontal pitch and curvature of the shadow mask are determined based on the characteristics of the deflection yoke 9 and electron gun 8 and the curvature of the inner surface of the panel 1 to secure the purity margin and direction change margin. The shadow mask designed with regard to the grouping rate is set in the cathode-ray tube such that the red, green and blue fluorescent materials are located on the screen of the panel 1 to exactly accord with the path of the electron beams.
In the cathode-ray tube constructed as above, the radius of curvature Rm of the shadow mask is basically determined to have a predetermined ratio to the radius of curvature Rp of the inner surface of the panel for realization of images. In a recently proposed cathode-ray tube having flat outer surface, as the radius of curvature of the inner surface of the panel becomes large, the radius of curvature of the shadow mask increases to make flat. Although strength of the shadow mask is not deteriorated when the ratio of the thickness of the effective area edge of the panel to that of its center is more than 2 in the conventional cathode-ray tube, the strength of the shadow mask of the flat cathode-ray tube is abruptly lessened due to a decrease in the thickness ratio of the effective area to the center of the panel.
The deterioration in the strength of the shadow mask causes howling that generates vibration of the curved surface of the shadow mask 3 and a deterioration in shock-resistance that results in permanent transformation of the curved surface of the shadow mask due to an external strong shock applied thereto during handling of the cathode-ray tube. Furthermore, the electron beams 11 emitted from the electron gun 8 are distorted while passing through the shadow mask 3 so that they cannot strike a target fluorescent material. Accordingly, the deterioration in the strength of the shadow mask brings about flickering and a decrease in the color purity, lowering the quality of the cathode-ray tube.
To solve the above problems, indentations were formed on the shadow mask 3 to make beads 14 to improve howling characteristic, as shown in FIG. 3. Otherwise, a damper wire 15 to which tensile force is applied is set on the shadow mask 3 to disperse energy, mitigating shocks, vibrations or amplitude of sound, as shown in FIG. 4. However, the method of FIG. 3 has a difficulty in coating of fluorescent materials on the inner side of the panel during manufacturing process because the beads 14 exist in the effective area. Furthermore, the fluorescent materials coated on the panel are not uniformly distributed locally, to generate distortion of images and to make people fill uncomfortable to see the screen.
In the method shown in FIG. 4, it is required that transformation of the shadow is prevented when the shadow mask 3 to which tensile force is applied is fixed to the frame 4, and the damper wire 15 can give uniform pressure to the entire surface of the shadow mask. This complicates the manufacturing procedure and increases manufacturing cost. In addition, the aforementioned methods of forming the beads 14 in the effective area and placing the damper wire 15 have a limit with respect to the howling characteristic though they have advantages in terms of the strength of the shadow mask.
There was also proposed a method in which the thickness ratio of the effective area edge to center of the panel 1 becomes relatively large to reduce the radius of curvature of the shadow mask, thereby improving the strength of the shadow mask. However, this method also causes breakage of cathode-ray tube in thermal processes, increases material cost of the panel, and generates a difference in brightness. As a result, it cannot satisfy target resolution and target color purity and deteriorates visual flatness.
It is, therefore, an object of the present invention to provide a shadow mask of a flat cathode-ray tube, capable of satisfying a target resolution while deterioration in the structural strength thereof is prevented.
To accomplish the object of the present invention, there is provided a shadow mask for a cathode-ray tube, which is placed in the rear of a panel whose outer surface is flat and whose inner surface has a predetermined curvature to select colors of incident electron beams, in which radiuses of curvature Rx, Ry, Rxe and Rye are determined based on appropriate ratios of them to a radius of curvature Rd, and the curvature of the shadow mask is decided by a combination of the radiuses of curvature Rx, Ry, Rd, Rxe and Rye, where Rx is the radius of curvature of the longer axis passing the center of the shadow mask, Ry is the radius of curvature of the shorter axis passing the center of the shadow mask, Rd is the radius of curvature of the diagonal axis passing the center of the shadow mask, Rxe is the radius of curvature of the end of the shorter side of the shadow mask, and Rye is the radius of curvature of the end of the longer side of the shadow mask.